Organic Acid Refining
Vegetable oils are typically obtained by pressing or extracting the oil seeds of plants such as corn or soybeans. Properly processed vegetable oils are suitable for use in many edible oil and fat compositions destined for human consumption. Such edible oils and fats include salad oils, cooking oils, frying fats, baking shortenings, and margerines. In addition to being widely used in edible oils and fats, vegetable oils are also increasingly utilized in important industrial products such as caulking compounds, disinfectants, fungicides, printing inks, and plasticizers.
Vegetable oils primarily consist of triglycerides, but several other compounds are also present. Some of these additional compounds, such as diglycerides, tocopherols, sterols, and sterol esters, need not necessarily be removed during processing. Other compounds and impurities such as phosphatides, free fatty acids, odiferous volatiles, colorants, waxes, and metal compounds negatively affect taste, smell, appearance and storage stability of the refined oil, and hence must be removed. Carefully separated, however, some of these additional compounds, particularly the phosphatides, are valuable raw materials. It is therefore important to select a vegetable oil purifying method that maximizes removal of impurities but does so in a way that least impacts the compounds removed.
Vegetable oil impurities are typically removed in four distinct steps of degumming, refining, bleaching, and deodorizing. Of these four steps, degumming removes the largest amount of impurities, the bulk of which are hydratable phosphatides. Refining primarily removes non-hydratable phosphatides, soaps created from the neutralization of free fatty acids, and other impurities such as metals. Bleaching then improves the color and flavor of refined oil by decomposing peroxides and removing oxidation products, trace phosphatides, and trace soaps. Soybean oil bleaching materials include neutral earth (commonly termed natural clay or fuller's earth), acid-activated earth, activated carbon, and silicates. Deodorizing is the final processing step and prepares the oil for use as an ingredient in many edible products including salad oils, cooking oils, frying fats, baking shortenings, and margerines. The deodorizing process generally comprises passing steam through refined oil at high temperature and under near vacuum conditions to vaporize and carry away objectionable volatile components.
Vegetable oil refining, also known as neutralization or deacidification, essentially involves removing free fatty acids (FFA) and phosphatides from the vegetable oil. Most refining operations employ either alkali refining (also termed caustic refining) or physical refining (also termed steam refining). Of these two refining methods, alkali refining predominates.
For either refining method, an optional but preferred first step is a conventional water degumming process. Degumming refers to the process of removing hydratable phosphatides and other impurities such as metals from vegetable oils. Crude vegetable oils contain both hydratable phosphatides (HPs) and non-hydratable phosphatides (NHPs). A simple degumming process comprises admixing soft water with the vegetable oil and separating the resulting mixture into an oil component and an oil-insoluble hydrated phosphatides component (frequently referred to as a "wet gum" or "wet lecithin"). The NHPs, generally considered to be calcium and magnesium salts of phosphatidic acids, are largely unaffected by water and remain soluble in the oil component.
Normally, refiners also must introduce chelating agents following degumming processes to remove metal compounds from crude vegetable oil, which typically contains calcium, potassium, magnesium, aluminum, iron and copper. Left in place, these metal impurities form salts of phosphatidic acid, thereby contributing to the NHP content. Moreover, metal contaminants, especially iron, can darken oil during deodorization, and even small amounts of iron that do not affect the oil's color can nevertheless dramatically reduce stability of refined oil.
Treating crude vegetable oil with soft water produces a degummed oil and a phosphatide concentrate containing the hydratable phosphatide fraction. This phosphatide concentrate subsequently can be removed from the degummed oil by a convenient method such as centrifugal separation. Phosphatide concentrates coming from centrifugal separation will generally contain up to about fifty percent by weight water, and typically will contain from about twenty-five to about thirty percent by weight water. In order to minimize chances of microbial contamination, phosphatide concentrates must be dried or otherwise treated immediately. Dried phosphatide concentrates can be profitably sold as commercial lecithin. Degummed oil is further refined to remove NHPs and other unwanted compounds.
Mineral acid also is sometimes added during the water degumming process to help minimize the NHP content of degummed oil. The acid combines with calcium and magnesium salts, enabling phosphatidic acids to migrate from the oil to the water phase, thus eliminating them from the crude oil. However, using mineral acid during degumming is inappropriate when seeking to recover gums intended for use as lecithin because the presence of mineral acid will cause darkening of the lecithin.
In alkali refining, free fatty acids and gums are removed from crude or degummed oil by mixing the oil with a hot, aqueous alkali solution, producing a mixture of so-called neutral oil and soapstock (also termed refining byproduct lipid), which is an alkaline mixture of saponified free fatty acids and gums. The neutral oil is then separated from the soapstock, typically by centrifugation. The soapstock has commerical value due to its fatty acid content but must be processed further in order to render it salable. The neutral oil is further processed to remove residual soap.
Soapstock is treated in a process called acidulation, which involves breaking or splitting the soap into separate oil and aqueous phases through addition of a mineral acid such as sulfuric acid to reduce the pH to approximately 1.5, followed by thorough heating and mixing. Because the aqueous phase is heavier than the oil phase, the acidulated soapstock is separated from the oil by gravity or centrifugation. The separated oil (termed acid oil) has essentially the composition of the neutral oil and is drawn off, washed with water to completely remove mineral acid and sludge, and sold, usually as an animal feed supplement. The remaining aqueous phase (termed acid water) is the final waste product and can either be used in other processes or neutralized before being discarded.
The alkali refining process has several drawbacks, however, mainly related to soapstock formation. One drawback is refining losses that occur due to the soapstock's emulsifying effect, wherein soapstock acts to take up a portion of the valuable neutral oil into the aqueous soapstock solution. To minimize such emulsification losses, the crude or degummed oil is usually heated to between 158.degree. F. and 194.degree. F. prior to being contacted with the hot alkali solution. However, heating will not completely prevent emulsions from forming, nor will it entirely break emulsions once formed. Centrifugation forces also are insufficient to completely break emulsions of neutral oil in soapstock.
Another drawback to alkali refining is losses that occur when a portion of the neutral oil undergoes alkaline hydrolysis, often referred to as saponification, to produce undesirable fatty acid salts. Allowing the alkali solution and the crude or degummed oil to remain in contact for only short times can minimize saponification losses but is often insufficient to remove impurities other than fatty acids, especially impurities such as phosphatides and metal compounds. Consequently, short contact times can make it necessary to conduct a second round of refining.
Yet another alkali refining drawback is that raw soapstock is troublesome to handle. Soapstock solidifies quickly upon cooling, so heated holding tanks and transfer lines are required to maintain temperatures above 140.degree. F. Elevated temperatures also are required to prevent fermentation. On the other hand, overly heating soapstock causes it to boil, producing excessive and troublesome foaming.
Still another drawback is the difficulty in disposing of the acid water created during soapstock splitting. Acid water is high in biochemical oxygen demand (BOD) and low in pH. Disposal regulations require at minimum that the acid water be neutralized before the waste can be dumped. Many states have much more stringent pollution controls, requiring often costly solutions to ensure effluent biodegradability.
Thus, alkali refining involves many processing steps and has many drawbacks. In attempting to address the problems associated with alkali refining, operators must simultaneously vary many factors including the amount of heat applied, the amount and concentration of alkali, and retention times. Successfully balancing all these factors is a complex and difficult task. Furthermore, successful balancing of factors nevertheless can leave the need for additional refining cycles.
An alternative to alkali refining is physical refining. Physical refining is a steam distillation process essentially the same as that used in conventional vegetable oil deodorization processes, where steam passing through vegetable oil vaporizes and carries away free fatty acids. The main advantage of physical refining over alkali refining is that no soapstock is generated. A second advantage is lower refining losses because there is no saponification of oil and no oil emulsifaction by soapstock.
Accordingly, there is significant interest in physical refining due to its economic advantages and friendliness compared to alkali refining. But because physical refining does not remove NHPs, any oils to be physically refined must be free of NHPs in order to ensure stable refined oils. Oils such as palm oil and tallow, which have low NHP content, can be successfully physically refined. But oils such as soybean oil and sunflower seed oil, which are relatively high in NHPs, are not commonly physically refined because the pre-refining step of water degumming does not remove NHPs. Moreover, physically refined soybean oils have only limited acceptance in the U.S. market due to their lack of flavor stability.
Thus, although present methods exist for refining vegetable oils, significant drawbacks remain. Alkali refining can substantially remove phosphatides and other impurities but presents economic challenges and water pollution concerns. Physical refining is economically and environmentally less challenging, but many vegetable oils including soybean oil which are high in NBPs cannot be acceptably physically refined. Consequently, there is a need for an improved process for purifying vegetable oils, and especially soybean oil.
A prior method for refining vegetable oils is disclosed in U.S. Pat. No. 2,410,926 comprising mixing strong aqueous acid solutions with oils, adding filter aid material to absorb the aqueous phase, and separating purified oil from the residue. Typically, 1.5 to 2 percent by weight of a saturated aqueous organic acid solution is added to crude vegetable oil. The amount of water is kept low and is limited to the amount that can be absorbed by the filter aid. Solid acids are used so that any excess above the solubility limit of the small amount of water employed will be retained in the filter cake, and will not pass with the refined oil. A disadvantage of this method is the need to filter out absorbent before the oil can be used.
U.S. Pat. No. 4,698,185 describes a vegetable oil refining method comprising the steps of finely dispersing an aqueous organic acid in a water-degummed oil to form an acid-in-oil dispersion, allowing the phases to remain in contact for a time sufficient to decompose metal salts of phosphatidic acid, adding a base to the acid-in-oil dispersion to increase pH to above 2.5 without substantial formation of soap, and finally separating the dispersion into an oil phase and an aqueous phase containing the phosphatides. The method typically utilizes 0.4 to 2 percent by weight of a 20 to 60 percent by weight organic acid solution and requires a degree of dispersion of at least 10 million droplets of aqueous acid per gram of oil. A disadvantage of this method is that pH basic materials like those used in alkali refining must be added in order to remove NHPs.
A somewhat similar method disclosed in U.S. Pat. No. 4,240,972 comprises adding an acid to a heated stream of crude vegetable oil and then immediately passing the mixture through a static mixer, intensively mixing for a fraction of a second to produce an acid-in-oil dispersion having acid droplets smaller than 10 microns, and then separating the dispersion into an oil phase and an aqueous phase containing the phosphatides. This method claims that producing ultrafine acid droplets eliminates the need for lengthy acid-oil contact times. However, acid-oil contact times of less than about 15 minutes are generally insufficient to sequester substantial amounts of metal impurities.
U.S. Pat. No. 4,049,686 describes a method of removing phosphatides from vegetable oils comprising dispersing an aqueous solution of an acid or acid anhydride into a heated vegetable oil, chilling the mixture to a temperature below 104.degree. F., adding a small amount of additional water, allowing the phases to remain in contact for a time sufficient to decompose metal salts of phosphatidic acid, and separating the dispersion into an oil phase and an aqueous phase containing the phosphatides. This method claims that by cooling to a temperature below 104.degree. F. and adding water in a step separate from the aqueous acid addition, the phosphatides are converted into a semi-crystalline phase. Disadvantages of this method include the need to add aqueous acid and water in separate steps and the need for a chilling step, both being aspects that increase overall processing complexity.
Further improvements in purifying vegetable oil have been sought, particularly with regard to obtaining purified vegetable oil low in free fatty acids, phosphatides, and other impurities such as metals in an environmentally friendly manner. The present invention relates to an improved process having advantages over those previously disclosed. In particular, this invention relates to a non-alkali process for purifying vegetable oil employing a dilute aqueous organic acid solution. This invention also relates to purified vegetable oil obtained by the improved process.
Lecithin Deodorizing
The term lecithin, from a true chemical sense, refers to phosphatidyl choline. However, as used by commercial suppliers, the term lecithin refers to a product derived from vegetable oils, especially soybean oil. In addition to phosphatidyl choline, lecithin derived from vegetable oil includes phosphatidyl ethanolamine, phosphatidyl inositol, phosphatidic acid, phosphatidyl serine, cyclolipids, and other components such as free sugars, metals and free fatty acids. Because they contain several phosphatidyl derivatives, commercial lecithins are often referred to as phosphatides or phosphatide concentrates. Other synonymous terms for phosphatide concentrates include wet gums or wet lecithin.
Lecithins are utilized in a broad variety of applications and perform an array of valuable functions. In edible compositions, lecithin contributes nutritional value and also can act as an emulsifying agent, surface active agent, anti-spattering-agent, or stabilizing agent. Lecithin can be used in technical applications as an anti-foam agent, dispersing agent, wetting agent, stabilizing agent, and as an anti-knock compound for gasoline formulations. In particular, in foods such as baked goods or margarine, lecithin is used as a dispersing agent, emulsifier, viscosity reducer and antioxidant. In cosmetics such as shampoos or skin lotions, lecithin is employed as a foam stabilizer, emollient, emulsifier, and wetting agent. In pharmaceuticals targeted for either topical or parenteral administration, lecithin functions as softening agent, carrier, emulsifier, and penetration enhancer. Lecithin also possesses unique release properties, and is useful in pan frying and pan grease formulations for baking, as well as in mold release formulations enabling casting forms to be easily removed.
However, lecithin can have an objectionable odor and flavor that is difficult to remove. Phosphatides easily oxidize when subjected to heating, and such oxidative products can contribute a bitter or rancid taste to lecithin. Heating of phosphatides can also induce formation of volatile decomposition products such as 4,5-dimethylisoxazole, which contributes an objectionable flavor to lecithin. Other volatile compounds such as isophorone, a contributor of objectionable odor, can form by an aldol condensation reaction involving solvent remaining from crude vegetable oil refining processes.
Decomposition rate, oxidation rate, and reaction rate all increase with increasing temperature. Furthermore, once objectionable volatile compounds are formed, high temperature conditions can cause chemical bonding of these compounds to the lecithin. These factors urge processing lecithin at low temperatures. Yet the desire to discourage microbial growth by minimizing lecithin's water content urges drying at high temperatures to ensure maximum moisture removal. Thus, there are conflicting considerations in lecithin processing and a need for a process that resolves these competing factors and produces optimal lecithin.
A prior method for preparing phosphatide concentrates is disclosed in U.S. Pat. No. 4,713,155 comprising treating crude vegetable oil with a hydrating agent, separating the resulting wet gum from the remaining oil by centrifuging, treating the separated wet gum in an electromagnetic field of 40,000-240,000 A/m, and drying the phosphatide concentrate at 122.degree. F. to 158.degree. F. under atmospheric pressure (760 mm Hg). This process is said to allow greater reduction in percent by weight moisture than earlier methods similarly operating at atmospheric pressure but not employing an electromagnetic field treatment step. A disadvantage of this method is that complicated equipment is necessary in order to reliably and safely deliver the required strength of electromagnetic field.
More conventional processes for removing moisture from phosphatide concentrates involve drying at high temperature under reduced pressure. Commercial lecithin is typically produced in a continuous process by drying phosphatide concentrates at a temperature of from 176.degree. F. to 203.degree. F. and at an absolute pressure of from about 50 mm Hg to about 300 mm Hg. Erickson, David R., Degumming and Lecithin Processing and Utilization, in Practical Handbook of Soybean Processing and Utilization 174, 179-80 (David R. Erickson ed. 1995); Van Nieuwenhuyzen, W., J. Amer. Oil Chem. Soc. 53:425 (1976). However, as noted above, processing lecithin at high temperatures risks increasing the concentration of objectionable volatile compounds, and further risks permanently fixing these objectionable compounds to the lecithin via chemical bonding.
Further improvements in processing lecithin have been sought, particularly with regard to obtaining lecithin having improved odor and flavor characteristics via reduced content of objectionable volatile compounds. The present invention relates to an improved process having advantages over those previously disclosed. In particular, this invention relates to a process for removing objectionable volatile components from lecithin using a combination of stripping steam and drying at low pressure and moderately elevated temperature. This invention also relates to lecithin obtained by the improved process.
Vegetable Oil Deodorizing
Deodorization is usually the final step in producing edible vegetable oils and fats. Vegetable oils such as soybean oil typically contain volatile impurities that can impart objectionable odor and taste. These volatile compounds must be removed in amounts sufficient to produce deodorized oil having consumer-preferred characteristics. Impurities imparting objectionable properties to vegetable oil include free fatty acids, aldehydes, ketones, alcohols, hydrocarbons, tocopherols, sterols, and phytosterols. Following removal, some of these impurities, especially tocopherols, can be recovered and profitably sold.
Vegetable oil deodorization typically involves a steam stripping process wherein steam is contacted with vegetable oil in a distillation apparatus operating at low pressure and a temperature sufficient to vaporize objectionable volatile impurities at the operating pressure. This process, commonly known as vacuum-steam deodorization, relies upon volatility differences between the vegetable oil and the objectionable impurities to strip the relatively more volatile objectionable impurities from the relatively less volatile vegetable oil. Vacuum-steam deodorization treatment also beneficially decomposes peroxides in the vegetable oil and removes other volatile compounds that may result from such decomposition.
In a typical vacuum-steam deodorizing process, vegetable oil is introduced into a distillation apparatus having a plurality of vertically spaced trays, commonly termed stripping trays. Within each stripping tray, steam injected into the vegetable oil entrains objectionable volatile impurities. The combined steam and entrained distillation vapors are then condensed into a distillate that can be disposed of or processed further to recover valuable materials. Condensation of distillation vapors, like those produced during deodorization, is generally accomplished under vacuum. The deodorized vegetable oil is subsequently cooled and is available for sale or further processing.
Vegetable oil consists of triglycerides composed mainly of three unsaturated fatty acids--oleic, linoleic, and linolenic--esterified on a glycerol molecule in various combinations. Fatty acids in cis form predominate in vegetable oil; however, these fatty acids can convert into trans form under the influence of heating. Edible fats and oils containing trans fatty acids present health concerns and are therefore undesirable.
Vegetable oil also contains tocopherols in amounts that vary depending on the plant from which the vegetable oil was extracted. Soybean oil in particular contains roughly 0.10 to 0.20 percent by weight tocopherols in mixed .beta., .gamma., .delta. and .epsilon. isomeric forms. These tocopherol isomers all demonstrate antioxidant properties in varying degrees and hence are valuable raw materials. Tocopherols are high boiling, however, and generally only vaporize at temperatures above about 500.degree. F. at pressures of about 10 mm Hg or less.
Steam temperature and the time for which the steam contacts the vegetable oil are both important variables directly influencing the types and amounts of objectionable volatile impurities that can be removed in a vacuum-steam deodorizing process. Increasing temperature more rapidly removes objectionable volatile impurities and tocopherols, but also produces more undesirable reactions such as trans fatty acid formation, fat splitting, and polymerization. Decreasing temperature reduces the rate of undesirable reactions such as trans fatty acid formation, but also removes less of the tocopherols and reduces deodorizer throughput capacity by requiring longer contact times. In addition to steam temperature, other variables influencing contact time include the amount of steam used relative to the vegetable oil, the quality of the vegetable oil, and the type of equipment used. Deodorizers utilizing typical steam sparger designs and operating at a uniform temperature of about 450.degree. F. to 510.degree. F. and a pressure of about 6 mm Hg. will usually require a 45 to 60 minute holding time.
Seeking maximum tocopherol recovery urges deodorizing at relatively high temperatures. Yet a desire to avoid trans fatty acid formation urges deodorizing at relatively lower temperatures and/or minimizing the time for which the oil experiences high temperature, which can lead to insufficient removal of objectionable volatile impurities. Accordingly, there is significant interest in a method of deodorizing vegetable oil that maximizes tocopherol recovery and removal of objectionable volatile impurities yet minimizes trans fatty acid formation.
A prior method for deodorizing vegetable oils is disclosed in U.S. Pat. No. 4,072,482 comprising passing superheated steam through oil located in a deodorizing tray of a continuous deodorizing apparatus operating at a temperature of 500.degree. F. and a pressure of from 2 to 6 mm Hg. Multiple deodorizing trays can be used, each operating at 500.degree. F. and each contacting steam with oil for a time of from 10 to 60 minutes. Operating at 500.degree. F., this process would be expected to remove significant amounts of tocopherol but at the same time produce deodorized oil containing a relatively high amount of trans fatty acids.
Further improvements in deodorizing vegetable oils have been sought, particularly with regard to obtaining deodorized vegetable oil low in trans fatty acids and low in tocopherol. The present invention relates to an improved process for deodorizing vegetable oils having advantages over those previously disclosed. This invention also relates to deodorized vegetable oil obtained by the improved process.